Coal blending has been utilized for many years to obtain a coal blend of desired properties from one or more individual coals. Particularly in coke making, it is important to have certain minimum properties of the coal that goes into the coke ovens to obtain satisfactory coke strength, coke stability, and other desired properties. Coal volatility or coal rank is one property that is a particularly important property of the coal in obtaining a desired coke strength and stability.
Metallurgical coke suitable for use in the very large blast furnaces being built today must have very high coke strength as indicated by hardness and stability. A minimum hardness of about 68 and a minimum stability of about 55 are generally essential. To achieve this quality of coke requires the use of expensive high quality coking coals having a high percentage of volatiles in the coal, a high fluidity and a relatively low percentage of inert components. Due to the desire to use less expensive coals and also due to the decreasing supply of the high quality metallurgical grade coals, great effort has been expended upon ways of achieving high strength coke using lower quality coals. See "The Critical Case of Coke", Journal of Metals, February 1972, pp. 32-34, incorporated herein by reference. See also U.S. Pat. No. 4,186,054, incorporated herein by reference.
In many of the methods of using lower quality coals to make coke, one of the essential features is blending the different quality of coals. For example, by blending high, medium and/or low volatile coal, it is often possible to obtain the desired volatility at reduced cost as compared to using straight medium volatile coal. Often a particular coal, such as a low volatile coal, is itself a blend of other coals. Thus, often the final coal blend is a blend of two or more separate coal blends. For purposes of simplicity herein, when coal blends are blended they are referred to simply as "coal" or "coals".
In the coke industry, blends of coals of differing ranks are commonly used to obtain the proper coal characteristics needed to produce high quality coke. The best coke is produced using a medium volatility bituminous coal. Unfortunately, current supplies of coals of this rank are scarce, and as a result, it is common to use a blend of low and high volatility bituminous coals to yield a coal mixture with the desired intermediate characteristics. The composition of such a blend is usually checked by determining the volatile matter and verified, if necessary by a petrographic analysis, which is a time consuming procedure.
One of the problems that freqently arises is that something goes wrong with the end use for which the coal is being used. For example, the coke strength becomes unacceptably low or unnecessarily high. Poor quality coke is often produced while coal samples are being sent to the laboratory for analysis. Days or weeks of good quality production are often lost while a determination of what went wrong is made. Not infrequently it is determined that something went wrong in the blending operation which needs correcting to bring the coals back to their desired proportions in the final blend.